1. Technical Field
The presently disclosed technologies are directed to a system and method for reducing the magnitude of the electrostatic field as a printing media substrate travels underneath a solid ink print head. The system and method described herein use an alternating current corona device to reduce the magnitude of the electrostatic field on a printing media substrate and decrease potential print quality defects.
2. Brief Discussion of Related Art
In order to ensure good print quality in direct to paper (“DTP”) ink jet printing systems, the media substrate must be held extremely flat in the print zone. Some proposed methods for achieving this use electrostatic tacking of the media substrate to a moving transport belt that is held flat against a conductive platen in the imaging zones. An undesirable side effect of electrostatic tacking of media substrates is the creation of a high electric field between the surface of the media substrate and the imaging heads (also referred to herein as print heads). As the media substrate travels in the printing zone, the high electrostatic field can affect the ink jetting, which results in print quality defects.
FIG. 1 depicts an exemplary prior art printing system. The media substrate (MS) is transported onto the hold-down transport using a traditional nip based registration transport with nip releases. As soon as the lead edge of the media substrate is acquired by the hold-down transport, the registration nips are released. A vacuum belt transport is used to acquire the media substrate (MS) for the print zone transport (PZT).
FIG. 2 depicts an alternate prior art method for media substrate acquisition wherein electrostatic forces are used to tack the media substrate (MS), e.g., paper, onto a transport belt (TB). The figure shows an exemplary media tacking method which is well known in the state of the art. The transport belt (TB) can be fabricated from relatively insulating (i.e., volume resistivity typically greater than 1012 ohm-cm) material. Alternatively, the transport belt (TB) can include layers of semi-conductive material if the topmost layer is made from relatively insulating material. If semi-conductive layers are included in the transport belt, the quantity “volume resistivity in the lateral or cross direction divided by the thickness of the layer” or “sheet resistance” is typically above 1010 ohms/square for any such included layers. The basic belt transport system includes a drive roll (D), tensioning roll (T) and steering roll (S). The transport belt material may be an insulator or a semiconductor. The basic media tacking is shown in the dashed box upstream of the print heads (PH). Two rolls (1 & 2) are used. Roll 1 is on top of the belt/media substrate and roll 2 is below the belt. A high voltage is supplied across roll 1 and roll 2 to produce tacking charges. Either roll 1 or roll 2 may be grounded. An optional blade (shown upstream of the rollers) can be used to enhance tacking by forcing the paper against the roll.
The media substrate, when tacked by electrostatic tacking methods, almost always produces an electric field. When the media substrate travels through the print zone, the high electric field resulting from the electrostatic tacking can interact with the ink ejection. This can frequently produce print quality defects. Accordingly, it is desirable to reduce the magnitude of the electric field when the media substrate passes the print heads in order to mitigate or eliminate print quality defects.